This project has defined the host's immune response to endogenous viral antigens present in spontaneous and chemically induced tumors and are active as transplantation antigens. These studies have resulted in conceptual advances of how the immune responses of the host interact with each other and the transplantation antigens in determining the outcome of the tumor-host relationship. The endogenous viral structural proteins have been shown to act as significant transplantation antigens. The transforming proteins encoded by the virus, but clearly of host cell origin, provide potential transplantatin antigens of great importance. The oncogenic sequences coding for these transforming proteins and variants of the transforming proteins themselves have in many cases been found in normal cells in several species including humans, and there now seems little doubt that they will have significance in the understanding and treatment of human cancer. The project is now focusing on the production of monoclonal antibodies to these proteins for their evaluation as transplantation antigens, their presence in tumors throughout the phylogenetic scale and their nechanisms of transformation and thus to define the means to biochemically or immunologically alter the events that lead to cell transformation.